The |nom_jour filter, when applied to a tag that returns a valid date, will display the common name of the day (Monday, Tuesday...).
Example: for an article published on 22-01-2008,
[(#DATE|nom_jour)] will display "Tuesday".
This filter can be used with an argument (either "abbr" or "initiale") which will modify the resulting string as an abbreviation or as an initial:
[(#VAL{2008-01-22}|nom_jour{abbr})] will display "Tue."
[(#VAL{2008-01-22}|nom_jour{initiale})] will display "T."